1. Field of the Invention
An aspect of the present invention relates to optical recording media and, more particularly, to a holographic recording medium in which interference patterns, formed from interference between signal light and reference light, are recorded as data and recording/reproducing methods and apparatuses for the holographic recording medium.
2. Description of the Related Art
Recently, holographic recording media used as storage media capable of overcoming physical limitations such as optical diffraction limitations and having large storage capability have been widely researched and developed. In the holographic recording medium, interference patterns formed from interference between signal light originated from an object and reference light are recorded as data. The holographic recording medium can store hundreds or thousands of binary-data pages in the same recording position. More specifically, the interference patterns (corresponding to the data) recorded in the holographic recording medium are obtained from the signal light which is modulated into binary-data pages and the corresponding incident reference light of which position, deflecting angle, wavelength, and the like vary with multiplexing methods. In order to reproduce the data recorded in the holographic recording media, reproducing-reference light having the same condition as that of the recording-reference light is incident on the holographic recording media. When the reproducing-reference light is incident, the recorded interference pattern diffracts the reproducing-reference light, so that the interference pattern can be demodulated into one binary-data page having original bright and dark pixel patterns.
Recording materials used for the holographic recording media include photopolymer and photo refractive crystal, and so on. Since the photopolymer is inexpensive and has high diffraction efficiency, the photopolymer has been widely used as a recording material for the holographic recording media. When photopolymer reacts with light, monomers thereof are polymerized, so that the refractive index changes. The photopolymer is constructed with the monomers, a polymeric binder, a sensitizing dye, and an initiator. The wavelength of light is selected by the sensitizing dye. The polymers are non-uniformly distributed in space according to the intensity of light, forming a high polymer density region and a low polymer density region. The non-uniform distribution of the polymer causes a non-uniform distribution in the refractive index. On the other hand, after desired interference patterns are projected on the holographic recording medium, the interference patterns are exposed to UV light to be cured so as for the reaction not to proceed. The process is called UV curing.
The refractive index and volume of the photopolymer changes with temperature at the time of the UV curing and the data recording. In turn, the changes in refractive index and volume cause a change in pitches between the recorded interference patterns, so that signals reproduced from the holographic recording medium may be degraded. Therefore, techniques for suppressing or compensating for the changes in refractive index and volume at the time of reproduction of the holographic recording medium have been widely researched.
An example of compensating for the changes in refractive index and volume according to the temperature includes a method of compensating for contraction of a recording layer at the time of reproduction by illuminating heat generating layers with a separate light source having a wavelength different from that of a recording light source disclosed in Japanese Patent Application No. 2004-139479. More specifically, the heat generating layers are disposed on both sides of the holographic recording layer. When the holographic recording layer is contracted, the heat generating layers are illuminated with heating laser having a wavelength different from those of recording and reproducing lasers so as to increase the temperature of the holographic recording layer. As a result, it is possible to compensate for the changes in refractive index and volume caused from the contraction and deformation of the holographic recording medium.
However, holographic recording media in the related art have problems in that production processes and costs increase due to the additional heat generating layers. In addition, there are some problems caused from a difference in physical characteristics at interfaces between the heat generating layers and the photopolymer. In addition, since an additional light source for illuminating the heat generating layers needs to be provided to a recording/reproducing apparatus for the holographic recording medium, there is a problem in the construction of the recording/reproducing apparatus becomes more complicated and power consumption increases.